There are sensor control systems that are used as terminal controls systems in which a communication line is connected between a microcomputer (corresponding to a main control device) and a plurality of sensors (corresponding to terminal devices), the microcomputer sends control data to the sensors by way of the communication lines, and the sensors send detected data to the microcomputer. Often, in recent times, information that is proximate to human senses is acquired using such a system, and various equipment is controlled to anticipate human needs. A large quantity of information must be handled in real-time (simultaneously) in order to achieve better control without causing human discomfort. Therefore, a large number of sensor control systems is used in a single piece of equipment. For this reason, large communication lines in such a system are not advantageous from the aspect of cost, and the microcomputer and sensors in the system are connected by simple communication means. In serial communication, for example, a small system can be built because the data is transferred serially one bit at a time, as indicated by the name.
Full-duplex and half-duplex systems are used as methods of carrying out bidirectional serial communication between a microcomputer and a sensor. The full-duplex system can constantly perform bidirectional communication, and a single communication line is required for each direction. If, for example, a single microcomputer independently controls four sensors, eight communication lines would be required.
The half-duplex system cannot simultaneously perform bidirectional communication, but the communication line can be shared. When all of the devices are connected by a shared communication line, a system can be built with few communication lines. However, since shared communication lines are used, data collisions (data conflicts) occur in cases in which transmission from a plurality of sensors occurs simultaneously. Patent Document 1 describes a technique for solving the conflict problem when half-duplex communication is used among a plurality of devices.
The system described in Patent Document 1 relates to a non-synchronous serial communication method that comprises at least one or more masters (corresponding to the main control device and microcomputer described above), and at least one or more slaves (corresponding to the terminal devices and sensors described above), and in which signal lines that function as data transmission lines are connectively formed.
In a configuration such as that shown in FIG. 1 of Patent Document 1, transmitted data experiences a conflict when simultaneously transmitted from a plurality of devices. In view of this situation, transmitting devices compare and verify their own transmission data and the received data received by way of the bidirectional communication line (data bus). When a transmitting device has determined that the own transmission data of the device is not in conflict on the data bus, or that the own data of the device has priority when there is a conflict, the transmitting device determines that transmission is successful and will continue communication thereafter. When the transmission device has determined that there is conflict, that the own data of the device does not have priority, and that the data bus cannot be accessed, the transmitting device will determine that transmission has failed and will thereafter withhold communication. Communication data conflicts are prevented and bus arbitration is carried out in this manner.
Patent Document 1: Publication of Japanese Patent Application No. 11-243405 (FIG. 1, Paragraphs 11 to 23)